1. Field of the Invention
The invention relates to an exhaust gas after-treatment system. The invention, furthermore, relates to a method for the exhaust gas after-treatment.
2. Description of the Related Art
Exhaust gas after-treatment systems of internal combustion engines are known, which comprise a particle filter and at least one exhaust gas after-treatment assembly arranged in flow direction of the exhaust gas, upstream of the particle filter. The exhaust gas after-treatment assembly, is, in particular, an oxidation catalytic converter for the oxidation of nitrogen monoxide (NO) into nitrogen dioxide (NO2). The term particle filter means both conventional particle filters through which the exhaust gas flows, as well as particle filters in which the exhaust gas flow is conducted along a separating structure.
In particular, when seen in flow direction of the exhaust gas flow upstream of the particle filter, an oxidation catalytic converter for the oxidation of NO into NO2 is positioned, NO oxidised into NO2 with the help of the residual oxygen O2 contained in the exhaust gas flow in the oxidation catalytic converter according to the following equation:2NO+O22NO2 
With this oxidation of nitrogen monoxide into nitrogen dioxide, the equilibrium of the oxidation reaction at high temperatures is on the side of nitrogen monoxide. This results in that the achievable component of nitrogen dioxide is greatly limited at high temperatures.
In the particle filter, the nitrogen dioxide extracted in the oxygen catalytic converter is converted with carbon-containing particles, so-called soot, collecting in the particle filter into carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2) and nitrogen monoxide (NO). In the process, a continuous removal of the carbon-containing particulate matter or of the soot accumulated in the particle filter takes place in the sense of a passive regeneration of the particle filter, wherein this conversion takes place according to the following equations:2NO2+C→2NO+CO2 NO2+C→NO+CO2C+2NO2→N2+2CO2 
In particular when with such passive regeneration of the particle filter no complete conversion of the carbon-containing particulate matter or of the soot embedded in the particle filter can take place, the carbon content or soot content in the particle filter increase, the particle filter then having a tendency towards clogging as a result of which ultimately the exhaust gas backpressure on an internal combustion engine upstream of the exhaust gas after-treatment system increases. An increasing exhaust gas backpressure on the internal combustion engine diminishes the power of the internal combustion engine and causes increased fuel consumption.
To avoid an increase of the carbon-containing particulate matter or of the soot in the particle filter and thus clogging of the same, it is already known from practice to provide particle filters with a catalytic coating. Platinum-containing coatings are preferentially employed here. The use of such particle filters with catalytic coating however can prevent charging the particle filter with carbon-containing particulate matter, i.e. with soot, only to an insufficient degree.
Furthermore it is known from practice to employ active regeneration of the particle filter to reduce the charging of a particle filter with soot. During such active regeneration of the particle filter, the exhaust gas temperature is actively increased for example by adding fuel to the exhaust gas flow to burn off carbon-containing particulate matter or soot particles, which have accumulated in the particle filter, via an exothermic reaction or oxidation of the hydrocarbons. Burning off the carbon with the help of oxygen in a particle filter thus takes place according to the following equation:C+O2→CO2 
During active regeneration of a particle filter by burning off the soot particles, a major increase in temperature up to 1000° C. can develop in the particle filter. During such a major temperature increase, damage to the particle filter can occur.